An acute contagious disease of the upper airways and lungs, caused by a virus, which rapidly spreads around the world in seasonal epidemics.
"The sudden outbreak of influenza swept through the city during winter, causing thousands to miss work due to the highly contagious viral infection."
In plain English: Influenza is a contagious virus that makes you feel very sick with symptoms like fever, coughing, and body aches.
"The doctor advised me to stay home while I recover from influenza."
Usage: Influenza refers specifically to the viral respiratory illness commonly known as the flu, rather than general colds or other infections. Use this term when discussing symptoms like high fever and body aches that distinguish it from milder upper airway illnesses.
The word entered English as a doublet of influence, originally referring to the supposed astrological power that was believed to cause epidemics. It derives from the Italian term for "influence," which itself comes from the Latin influentia.